Iodine-125 (.sup.125 I) is a radioactive isotope of iodine with a relatively long half-life of 60 days. This material is used for medical diagnostic studies and for medical and biological research. This iodine isotope is valuable because the radiation it emits is less damaging than that from other isotopes of iodine.
It is known to produce such material by irradiating .sup.124 Xe with thermal neutrons, according to the following scheme: ##STR1## .sup.125 I decays to form .sup.125 Te or may be converted to .sup.126 I which decays to .sup.126 Te, as follows: ##STR2## Supplies of .sup.125 I isotope are limited and there is an increasing demand for this material. Iodine-126 that is present with .sup.125 I is a contaminant. Because of the emission of more damaging radiation by .sup.126 I, the Food and Drug Administration, U.S.A., requires that .sup.125 I for use in the human body contains less than 5 parts per million of .sup.126 I.